In semiconductor devices, insulating layers such as silicon dioxide (SiO2), doped oxide such as BPSG, and silicon nitride, are used to electrically separate conductive layers, such as doped polysilicon, metals, refractory metal silicides, etc. High aspect ratio contact (HARC) etching is a key process for forming contact hole interconnections through insulating layers to an active device area or conductive layer in high density integrated devices. HARCs require etching processes that produce a vertical profile and a defined critical dimension (CD). Another application for HARC features in dielectrics is the formation of capacitor or container structures in stacked capacitor DRAM.
In a typical plasma etching, a substrate is positioned on a chuck in a gas chamber, an etchant gas is introduced into the chamber, and the chamber pressure is reduced. An energy source/power supply creates a charged electric or electromagnetic field through electrodes positioned within the chamber to energize the etchant gas into a plasma state. The etchant gas is transformed into a disassociated mixture of uncharged neutrals as well as electrons and positive ions. Typically, the positive ions are accelerated to the substrate by a radio frequency (RF) biased electrode sheath providing directionality for forming vertical contact hole profiles, and the chuck supporting the substrate acts as a bottom electrode and can be biased by a second RF power source. The ions react with the substrate thereby removing the exposed material from the semiconductor device.
Generally, standard processes for etching through silicon dioxide (SiO2) to underlying silicon and/or silicon nitride use fluorocarbon gas plasmas. The dissociation of the fluorocarbon molecules by the action of plasma generates active radicals and/or ions that act on the SiO2 substrate. For example, in some high density plasmas, CF+, CF2+ and CF3+ ions generated from CF3 and other CxFy radicals (where x is up to 11 and y is up to 15) are dominant etching ions for SiO2, with Ar+ ions sputtering CFx films on oxide, and less fluorinated radicals (e.g., CF2 and CF) are adsorbed on the sidewalls and bottom surfaces in the contact hole of the SiO2 during the etching process and polymerize to form a nonvolatile fluoropolymer layer that inhibits etching by the ions. Precise control of the balance between adsorbing radicals and etching ions during SiO2 contact plasma etching is important to simultaneously passivate the sidewalls of the opening and extend the etch front at the bottom of the opening. However, such control is difficult when using conventional fluorocarbon chemistries.
Problems including bowing and/or twisting of the openings often occur during HARC etches. FIGS. 1 and 2 illustrates a substrate fragment (e.g., wafer) designated generally as 10, showing formation of a contact hole 12 in a dielectric layer 14 to an underlying substrate layer 16 by prior art etching methods. Bowing, illustrated in FIG. 1, is generally formed by the reaction of free fluorine which accumulates on the sidewalls 18 of the contact opening 12 during etching (arrows ↓↓↓) and laterally attacks and etches the exposed sidewalls producing a characteristic bowing 20. During typical HARC etches, charge buildup along the sidewalls 18 of a narrow and deep opening can deflect incoming ions causing changes in the trajectory of those ions. Twisting, illustrated in FIG. 2, can be caused by asymmetric polymer deposition 22 along the sidewalls 18 during etching in conjunction with feature charging, which can cause a deflection of incoming etching ions and a changed trajectory (arrow) of the ions, resulting in the twisting or bending of the contact hole 12 from a perfectly vertical profile with the hole tending toward one side or the other. Twisting of the contact hole can lead to misalignment and an imperfect contact between a subsequently deposited conductive metal and an active area landing region 24, for example, in the underlying substrate 16. Deviations such as bowing and twisting can also result in a non-vertical contact hole and shorting of the contact to an adjacent contact or other structure.
It would be useful to provide a method and etchant gas for etching high aspect ratio openings in silicon oxide layers that overcomes these or other problems.